We also required a catchall without mailbox solution for Dovecot/Sieve on ISPconfig 3. I found that these steps work perfectly.

1. Create a new mailbox for the domain (e.g.: [email protected]).
2. Create a catchall for the domain that points to new mailbox.
3. Edit mailbox and go to the "Custom Rules" tab. Enter these two rules:

If I use these rules, an email sent to "[email protected]" is redirected to "[email protected]" and discarded. No messages are saved in the catchall mailbox! You MUST include the "stop;" line or Sieve will NOT discard the message after forwarding.

Suppose your catchall mailbox is "[email protected]". Login to the mail server that hosts the mailbox and type the following to view the entire Sieve filter for this mailbox.

Notice the "keep;" line added by ISPconfig 3 at the end of the file? If you do not specify "stop;" to terminate the Sieve script, Dovecot will execute the "keep;" command and place a copy of the message in your catchall mailbox.